eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
Available online at www.expresspolymlett.com
DOI: 10.3144/expresspolymlett.2007.10
Peroxydisulfate initiated synthesis of potato
starch-graft-poly(acrylonitrile) under microwave irradiation
V. Singh1*, A. Tiwari2, S. Pandey1, S. K. Singh1
1Department
2Division
of Chemistry, University of Allahabad, Allahabad-211 002, India
of Engineering Material, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110 012, India
Received 21 October 2006; accepted in revised form 27 December 2006
Abstract. Potato starch-graft-poly(acrylonitrile) could be efficiently synthesized using small concentration of ammonium
peroxydisulfate (0.0014 M) in aqueous medium under microwave irradiation. A representative microwave synthesized
graft copolymer was characterized using Fourier Transform Infrared Spectroscopy, X-ray Diffraction, Scanning Electron
Microscopy and Thermo gravimetric Analysis. Under microwave conditions oxygen removal from the reaction vessel was
not required and the graft copolymer was obtained in high yield using very small amount of ammonium peroxydisulfate,
however using the same amount of ammonium peroxydisulfate (0.0014 M) on thermostatic water bath no grafting was
observed up to 98°C (even in inert atmosphere). Raising the concentration of the initiator to 0.24 M resulted into 10% grafting at 50°C but in inert atmosphere.
The viscosity/shear stability of the grafted starch (aqueous solution) and water/saline retention ability of the microwave
synthesized graft copolymer were also studied and compared with that of the native potato starch.
Keywords: polymer synthesis, biopolymer, starch, peroxydisulfate, microwave irradiation
1. Introduction
ciency [13] and (b) production of the starch-based
superabsorbent hydrogels [14] by alkaline hydrolysis of the starch-graft-poly(acrylonitrile). So far
voluminous reports are available on the synthesis
of starch-graft-poly(acrylonitrile) using redox initiators where inert atmospheric condition is the
requirement since O2 is a very potent inhibitor for
most of the common vinyl monomers.
Microwave irradiation [15–19], as efficient thermal
energy source constitutes a very original method of
heating materials, different from the classical ones.
Main advantage is that it results in almost instantaneous bulk heating of materials in a homogeneous
and selective manner. Under microwave irradiation
grafting and homopolymerization have been studied without initiators [3, 4] or in the presence of
very low concentration of initiator [20, 21] and this
prompted us to undertake microwave promoted
Starch which is a low cost abundantly available
renewable biopolymer on modification may
develop properties comparable to synthetic petroleum-based polymers [1, 2] Chemical grafting [3–
7] is one of the popular methods for modifying
structure and properties of biopolymers. Vinyl
grafting on to the polysaccharides improves their
shelf life against biodegradation, thermal stability
and metal ion binding ability and therefore grafted
polysaccharides have a wide range of industrial
utility [8]. Considerable amount of research has
been published on ceric ion initiated graft copolymerization of acrylic monomers onto starch [9–11].
Potassium permanganate [12] has also been used as
initiator to graft poly(acrylonitrile) on to starch.
Most of the studies have been focused on grafting
of acrylonitrile due to: (a) its superior grafting effi*Corresponding
author, e-mail: [email protected]
© BME-PT and GTE
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Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
grafting of poly(acrylonitrile) onto potato starch
using small concentration of ammonium peroxydisulfate as initiator.
Grafting ratio [%G] =
W1 − W0
·100
W0
Grafting Efficiency [%E ] =
2. Experimental Section
W1 − W0
·100
W2
(1)
(2)
Where W1, W0, and W2 denote, respectively, the
weight of the starch-graft-poly(acrylonitrile) (S-gPAN) the weight of original starch, and weight of
the acrylonitrile used.
2.1. Materials and Instruments
A Kenstar (Model No. MOW 9811) domestic
microwave oven with a microwave frequency of
2450 MHz and a power output from 0 to 1200 W
with continuous adjustment was used for all the
experiments. The starch sample (Qualigen, Extra
pure) containing 30% amylose and 70% amylopectin was used without purification. Acrylonitrile (Lancaster, synthesis grade) was distilled in
a stream of nitrogen before use. Ammonium peroxydisulfate (Merck) was used without further purification and double distilled water was used for the
grafting reactions. Infrared (IR) spectra were
recorded on a Nicolet 5700 of FTIR spectrophotometer using KBr pellet. X-Ray Diffraction (XRD)
was carried out on Rigaku D/MAX-2200 X ray
powder diffractometer. TGA was done on Perkin
Elmer SII Dimond TGA/DTA at a heating rate of
10°C per min under nitrogen atmosphere. For the
characterization, sample with maximum grafting
was used. The viscosity measurements (of the
aqueous solutions) were made on Brookfield
LVDVE viscometer at 25°C using small sample
adapter (spindle no S-18).
2.3. Graft co-polymerization of starch on
thermostatic bath
Starch (0.1 g) was dissolved in 10 ml of hot distilled water in a two-necked flask. To this acrylonitrile (0.17 M) was added and the total volume was
made up to 25 ml. The reaction mixture was purged
with purified nitrogen for about 30 min and thermostated at 50°C. Ammonium peroxydisulfate
(0.0014 M) was added to the reaction flask and this
time was taken as zero time and the graft copolymerization was carried out for one hour. Reaction
mixture was finally poured in to 100 ml of DMF [3]
to precipitate the graft copolymer. The copolymer
was repeatedly washed with DMF to remove the
adhered PAN if any and dried. The above experiment was repeated at 98°C. Under identical conditions the reaction was also performed in presence of
O2 at both the temperatures. Grafting was also done
with different persulfate concentration ranging
from 0.0020–0.24 M at 50°C.
2.2. Acrylonitrile grafting on to starch under
microwave irradiation
2.4. Hydrolysis of grafted starch in aqueous
alkali
A calculated amount of the starch was dissolved in
minimum required amount of warm distilled water
in a 150 ml open necked flask. To this solution, calculated amount of the ammonium peroxydisulfate
and acrylonitrile were added and the total volume
was made up to 25 ml. The flask was exposed to
fixed microwave power and exposure time. The
graft copolymer was separated from poly(acrylonitrile) (PAN) by pouring the reaction mixture into
100 ml dimethylformamide (DMF) [3]. The copolymer was repeatedly washed with DMF to
remove adhered PAN. The grafting experiment was
also repeated in presence of hydroquinone (20 mg).
The grafting ratio and grafting efficiency were calculated from the increase in weight in the following
manner [8].
Starch-graft-poly(acrylonitrile) (2 g on dry basis)
was dispersed in 2% NaOH at 100°C for 1.5 h.
After hydrolysis [8] the sample was precipitated in
600 ml methanol, washed with methanol followed
by ethanol, dried and weighed.
2.5. Water and saline (1% NaCl) retention
The sample (0.5 g on dry basis) was swollen in
100 ml distilled water for 30 minutes. The suspension was poured into a G-4 sintered glass filter at
93.32 kPa pressure. Water retention [8] was calculated as gram of water absorbed per gram of dry
material. Similarly saline retention [8] capacity was
determined using 1% aqueous sodium chloride
solution.
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Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
2.6. Viscosity measurement
3.1. Characterization of the grafted starch
A weighed quantity of the samples were dissolved
separately (Starch was dissolved in hot distilled
water by stirring while the graft copolymer and
saponified graft copolymer were dissolved in cold
water by soaking them overnight) and then the
resulting solutions were made up to desired concentrations and agitated vigorously for about
15 min till they became viscous and homogeneous.
Viscosities of starch, starch-graft-poly(acrylonitrile) and saponified starch-graft-poly(acrylonitrile)
were measured after different time intervals.
A representative microwave synthesized graft
copolymer sample (sample with maximum grafting
ratio), was characterized using IR, XRD and TGA.
IR spectrum (Figure 1) of pure starch has a broad
strong absorption at 3310–3425 cm–1 (O–H stretching) and a peak at 2947 cm–1 (C–H stretching)
while IR spectrum of starch-graft-PAN has additional absorption peaks at 2242 cm–1 (–CN stretching) and 1453 cm–1 (CH2 deformation vibration)
which can be attributed to grafted PAN chains at
starch backbone. Physical blends of gum and PAN
after selective removal of PAN with DMF showed
no absorption in –CN stretching and –CH2 bending
region. This substantiates the formation of the graft
copolymer.
TGA of Starch (Figure 2) and starch-graft-PAN
showed that grafted starch to be more thermally stable than the pure Starch. Up to 650°C, 49% loss in
total weight was observed for grafted starch in
comparison to 58% weight loss in starch.
Comparison of XRD of the starch, poly(acrylonitrile) (PAN) and starch-graft-PAN further confirms
grafting (Figure 3). XRD spectra of the grafted
starch showed increased crystallinity in the region
of 2θ 28–32° due to the presence of PAN grafts on
starch backbone, while peak at 2θ 18° originally
present in the starch has been significantly reduced
after grafting.
Grafting is also evidenced by the SEM picture of
the starch, picture clearly shows change in the surface topology of the starch after grafting due to the
growing poly(acrylonitrile) grafts (Figure 4).
3. Results and discussion
Using low concentration of the ammonium peroxydisulfate, poly(acrylonitrile) could be very efficiently grafted on to the potato starch under
microwave irradiation. Maximum grafting ratio
and efficiency (225% and 98% respectively) were
obtained at 0.17 M acrylonitrile, 0.0014 M
(NH4)2S2O8, 0.1 g potato starch, 70 seconds exposure and 1200 W microwave power keeping total
reaction volume fixed at 25 ml. The temperature of
the reaction mixture just after the microwave exposure was measured to be 98°C. However at same
concentrations of acrylonitrile (0.17 M) and starch
(0.1 g/25 ml) on thermostatic water bath at 98°C,
0.0014 M peroxydisulfate was unable to trigger
graft copolymerization (both in the nitrogen as well
as in oxygen atmosphere) indicating that under
conventional conditions, 0.0014 M peroxydisulfate
is unable to furnish sufficient primary free radicals
required for the graft copolymerization. However
when the peroxydisulfate concentration was raised
to 0.24 M (in conventional procedure), 10% grafting in nitrogen atmosphere was observed.
O2 is a very potent inhibitor for most of the common vinyl monomers since it combines with active
primary free radicals to give relatively inactive radicals, which are incapable of propagating the chain
reaction. Thus the reaction media are usually thoroughly purged with inert gas to remove O2, otherwise the reaction may not work, however under
microwave conditions presence of O2 does not
effect the normal graft copolymerization reaction.
It can be assumed that under the influence of
microwaves the oxygen reversibly combines with
the primary free radicals and thus is unable to
inhibit the chain propagation; however more investigations are required on this issue.
Figure 1. IR spectra of starch (A) and starch-graftpoly(acrylonitrile)
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Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
Figure 2. TGA of starch (A) and starch-graft-poly(acrylonitrile) (B)
Figure 4. SEM picture of starch (A) and starch-graftpoly(acrylonitrile)(B)
Figure 5. Generation of primary free radiacals by
(NH4)2S2O8 under microwave irradiation
Figure 3. XRD of starch (A), starch-graft-poly(acrylonitrile) (B) and poly(acrylonitrile)
3.2. Mechanism of grafting
Since the grafting was not observed when the radical scavenger hydroquinone was added to the reaction mixture, a free radical mechanism for the
grafting is the most probable and is proposed as
under.
The grafting is being carried out in aqueous
medium and the water being polar, absorbs
microwave energy. This results in dielectric heating
of the reaction medium. Microwaves are also
reported to have the special effect [15] of lowering
of Gibbs energy of activation of the reactions.
These two effects cause quick decomposition of
peroxydisulfate into sulphate ion radicals
(Figure 5).
Figure 6. Graft copolymerization initiated by primary free
radiacals
–
SO4 . and OH. are the primary radicals, generated
in the above sequence of reaction, and are
expressed as R. in Figure 6. They initiate the vinyl
polymerization as the vinyl polymerization is
54
Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
reported to be faster than the H abstraction from the
polysaccharide starch backbone [5]. The macro
radical (SO.) may be generated by abstraction of H
by the growing vinyl polymer radical, which may
add onto the vinyl monomer (M) generating new
radical SOM. and this chain will grow till it combines with other such chains to give the graft
copolymer (Figure 6).
Since under identical reaction conditions, 0.0014 M
persulfate could not initiate graft copolymerization
in conventional grafting method (on thermostatic
water bath) even at 98°C, some microwave effect
cannot be ruled out in the microwave accelerated
grafting procedure.
3.3. Determination of optimal grafting
conditions under microwave irradiation
Figure 8. Effect of exposure time on %G and %E; at
[(NH4)2S2O8] 0.001 M; [acrylonitrile] 0.17 M;
[starch] 0.1 g/25 ml and 1200 W microwave
power
The microwave power, exposure time and concentration of the reactants (ammonium peroxydisulfate, acrylonitrile and starch) were varied keeping
the total reaction volume fixed at 25 ml. The maximum grafting ratio and efficiency that could be
reached were 225% and 98% respectively.
3.3.2. Effect of exposure time
Grafting increased with increase in exposure time
and was most for 70 sec exposure at fixed concentration of acrylonitrile (0.17 M), (NH4)2S2O8
(0.0010 M), starch (0.1 g), microwave power
(1200 W) and reaction volume (25 ml) (Figure 8).
On increasing the exposure time, grafting ratio and
efficiency increased. This may be due to the availability of more microwave energy resulting in to
extra SOM.n radicals for the graft copolymerization.
3.3.1. Effect of microwave power
On increasing microwave power from 240–1200 W,
grafting ratio and efficiency increased (up to
1200 W microwave power) at fixed concentration
acrylonitrile (0.17 M), (NH4)2S2O8 (0.0010 M),
starch (0.1g), exposure time (30 sec) and reaction
volume (25 ml) (Figure 7). Increase in %G and %E
with increasing microwave power may be due to
the formation of more M.n and SO. radicals, resulting into more SOM.n radicals for graft co-polymerization.
3.3.3. Effect of monomer concentration
Grafting ratio increased on increasing the monomer
concentration after 0.17 M; however efficiency
decreases under the fixed concentration of starch
(0.1 g/25 ml) at 1200 W microwave power and
70 sec exposure in 25 ml reaction volume
(Figure 9). The increase in %G may be due to the
formation of more M.n, generating more grafting
sites and due to availability of extra monomer for
grafting. Decrease in efficiency on increasing the
monomer concentration beyond 0.17 M may be
probably due to more homopolymer formation.
3.3.4. Effect of ammonium peroxydisulfate
concentration
Figure 7. Effect of microwave power on %G and %E at
[(NH4)2S2O8]·0.001 M; [acrylonitrile] 0.17 M;
[starch] 0.1 g/25 ml and exposure time 30 sec
The effect of ammonium peroxydisulfate on the copolymerization was studied in the concentration
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Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
Figure 9. Effect of acrylonitrile concentration on %G and
%E at [(NH4)2S2O8] 0.001 M: [starch]
0.1 g/25 ml: 1200 W microwave power and
70 seconds exposure
Figure 11. Effect of starch concentration on %G and %E
at [(NH4)2S2O8] 0.0014 M, [acrylonitrile]
0.17 M, 1200 W microwave power and 70 sec
exposure
3.3.5. Effect of starch concentration
The effect of Starch concentration was studied in
the range of 0.1–0.2 g/25 ml at fixed concentration
of ammonium peroxydisulfate (0.0014 M), acrylonitrile (0.17 M), microwave power (1200 W),
exposure time (70 sec) and reaction volume (25 ml)
(Figure 11). It was found that both %G and %E
decrease with the increase in the concentration of
starch, which may be due to the increase in the viscosity of the reaction medium causing hindrance of
the normal reaction and also due to decrease in the
monomer: starch ratio.
Overall maximum grafting ratio and efficiency that
could be achieved was 225% and 98% respectively
at 0.0014 M ammonium peroxydisulfate; 0.17 M
acrylonitrile; 0.1 g/25 ml starch, 1200 W microwave power, 70 sec exposure keeping total reaction
volume fixed at 25 ml.
Figure 10. Effect of ammonium peroxydisulfate concentration on %G and %E at [acrylonitrile]
0.17 M; [starch] 0.1 g/25 ml; 1200 W
microwave power and 70 seconds exposure
range 0.0010–0.0014 M at fixed concentration of
acrylonitrile (0.17 M), starch (0.1 g/25 ml) at
1200 W microwave power and 70 sec exposure in
25 ml reaction volume (Figure 10). It was observed
that the %G and %E was not significantly affected
by change in peroxydisulfate concentration in the
studied concentration range. This may be due to
very fast reaction taking place under the influence
of the microwaves so minor change in the initiator
concentration does not affect grafting.
3.4. Viscosity
Viscosity of 1% solution of starch, starch-graftPAN (%G-225) and saponified starch-graft-PAN
were found to be 0.008 Pas-s, 0.022 Pas-s and
0.203 Pas-s respectively (Table 1). The viscosity of
the pure starch solution was found prone to biodegradation and its viscosity was lost slowly on
Table 1. Viscosity of starch, starch-graft-PAN and saponified starch-graft-PAN with time at 25°C
No.
Sample
%G
1.
2.
3.
Starch
S-g-PAN
Saponified S-g-PAN
–
225
225
Initial
0.008
0.022
0.203
Viscosity of 1% solutions in Pas-s after different time intervals
8h
12 h
24 h
48 h
86 h
128 h
0.006
0.004
0.003
0.003
0.003
0.003
0.022
0.022
0.022
0.022
0.022
0.022
0.204
0.205
0.206
0.209
0.209
0.209
Where S-g-PAN stands for starch-graft-poly(acrylonitrile)
56
250 h
0.003
0.022
0.209
Singh et al. – eXPRESS Polymer Letters Vol.1, No.1 (2007) 51–58
Table 2. Water and saline retention for starch, starch-graft-PAN and saponified starch-graft-PAN at 25°C
No.
1.
2.
3.
Sample
Starch
S-g-PAN
Saponified S-g-PAN
%G
–
225
225
Water retention [g/g]
18.01
11.93
58.96
Saline (1% NaCl) retention [g/g]
15.43
8.17
49.31
Where S-g-PAN stands for starch-graft-poly(acrylonitrile)
Table 3. %G and %E in microwave and conventional methods in atmospheric and inert conditions at different peroxydisulfate concentration using starch (0.1 g/25 ml), acrylonitrile (0.17 M), total reaction volume 25 ml
No.
1.
2.
Microwave method
Conventional method
Concentration Persulfate in M
Condition
0.0014
Atmospheric
Atmospheric
0.0014
N2 atm
0.0014
Atmospheric
0.0014
N2 atm
0.0014
N2 atm
0.014
0.028
N2 atm
0.14
N2 atm
0.24
N2 atm
standing while the grafted and saponified grafted
starch solutions retain their viscosity even after
250 h (Table 1).
Temperature in °C
98
%G
225
%E
98
50
50
98
98
50
50
50
50
Nil
Nil
Nil
Nil
Nil
Nil
Nil
10
Nil
Nil
Nil
Nil
Nil
Nil
Nil
4
sure keeping total reaction volume fixed at 25 ml.
On grafting viscosity and shear stability of starch
solutions increase while water and saline retaining
ability decrease.
3.5. Water retention and saline retention
Acknowledgements
The water retention property is due to the interaction of the hydroxyl groups of the starch through
hydrogen bonding. The grafting of the vinyl
monomers onto the starch occurs through the
hydroxyl groups of its backbone thereby decreasing
the number of available hydroxyl groups for water
and saline retention. Thus water and saline retention ability of the graft copolymer proportionally
decrease with the increase in the grafting ratio
(Table 2 and 3). On hydrolysis with aqueous alkali,
the –CN groups on the grafted chains get
hydrolyzed to –CONH2 and –COOH groups and
this increases water-binding sites in the saponified
starch-graft-PAN and thereby a larger volume of
water is bonded.
Authors are thankful to Department of Science and
Technology, New Delhi, India for generous financial support to carry out this work.
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